Alumina is a well known catalyst support and a catalyst. It is also well known that the properties of alumina can be modified in various ways such as by cogelling with silica to form a silica-alumina. For example, U.S. Pat. No. 4,758,330 discloses a silica-alumina support prepared by forming a hydrogel of alumina and adding to the hydrogel an alkali metal silicate. Additionally, U.S. Pat. No. 4,806,513 discloses preparing an alumina with a surface coating of silica and further treated with fluorine.
In contrast to this art, applicants have prepared a composition in which some of the aluminum atoms have been removed from the alumina lattice and silicon atoms have been added to the composition. Applicants have also found that the silicon can be incorporated into the alumina framework. It has additionally been found that the basic structure of the starting alumina is maintained in the silicon enhanced alumina (hereinafter SEAL). Finally the SEAL composition also contains fluorine.
The SEAL compositions of this invention are prepared by contacting an alumina with a fluorosilicate salt at reaction conditions to remove some of the aluminum atoms and enhance the alumina with silicon. Although the prior art discloses the use of fluorosilicate salts, it is in regard to treating zeolites. Thus, U.S. Pat. No. 4,576,711 discloses contacting a Y-zeolite with an aqueous solution of ammonium hexafluorosilicate. Similarly, U.S. Pat. No. 4,503,023 also discloses dealumination of zeolites, in this case LZ-210. Other relevant prior art includes:
U.S. Pat. No. 4,753,910 which discloses using a water soluble fluoride during or after the aluminum removal step in order to solubilize the aluminum fluoride which is produced during the aluminum removal (dealumination step).
U.S. Pat. No. 4,711,770 discloses inserting silicon atoms into the crystal lattice of an aluminosilicate zeolite by contacting the zeolite with a fluorosilicate salt at a pH of about 3 to 7 and at a rate to preserve at least 60% of the crystallinity of the zeolite. This patent also discloses materials which have defect sites in the framework.
U.S. Pat. No. 4,597,956 discloses a method of removing aluminum fluoride byproducts by contacting the aluminosilicate with a soluble aluminum compound such as aluminum sulfate.
There is no mention in any of these references that one could prepare a crystalline SEAL composition by treatment of a crystalline alumina with a fluorosilicate salt. Applicants are the first to have synthesized such a novel composition. The SEAL compositions are useful as catalysts in various hydrocarbon conversion processes.